1. Field of the Invention
This invention deals generally with digital enhancement of environmental satellite imagery of airborne dust layers over both ocean and land backgrounds.
2. Description of the Related Prior Art
The detection and tracking of airborne dust from satellite has been of long-standing interest to both the Navy and academia alike, and there is no shortage of papers treating the subject (e.g., Shenk, W. E., and R. J. Curran, 1974: The detection of dust storms over land and water with satellite visible and infrared measurements, Mon. Weather Rev., 102, 830-837. Carlson, T. N., 1978: Atmospheric turbidity in Saharan dust outbreaks as determined by analyses of satellite brightness data, Mon. Weather Rev., 107, 322-335. Legrand, M., M. Desbois, and K. Vovor, 1987: Satellite detection of Saharan dust: optimized imaging during nighttime, J. Climate, 1, 256-264. Jankowiak, I., and D. Tanre, 1992: Satellite climatology of Saharan dust outbreaks: method and preliminary results, J. Climate, 5, 646-656.) These earlier works make use of various spatial and spectral contrast signatures to identify and retrieve properties of dust against land and ocean backgrounds.
A recent study (R. Cantu; “The Role of Weather in Major Naval Aviation Mishaps”, MS Thesis, 106 pp., NPS Monterey) finds that poor visibility has factored into approximately 54% of Navy aircraft “Class-A” mishaps, for an estimated annual loss of $51 million. Of these mishaps it was reported that 56% were preventable had better forecasting and/or observational tools been available. The current Dust Enhancement Product represents a paradigm shift in the way dust is observed from the space platform, and is directly applicable to mitigating this multi-million dollar liability.
As noted in the reference listing above, several alternative methods exist for enhancing dust over land and water. Single-channel radiometer imagery with scaling thresholds chosen to enhance a small dynamic range of temperature or reflectance values has been applied to visible and infrared geostationary imagery. The resultant enhancements, usually only of marginal visual quality, are highly dependent on the thresholds chosen and hence not suitable for operations where optimal values for these thresholds may change dynamically over space and time. Combinations of single-channel visible and infrared data is a marked improvement over the scaling described above, but still suffers cloud/dust ambiguity for lack of multi-channel visible data (i.e., the basis for the over-water Dust Enhancement Product, which requires blue and short wave infrared channels). The Empirical Orthogonal Function (EOF, or sometimes referred to as “principle component”) method has been shown to do a good job of enhancing dust over land and water. However, results produced by this method again are highly dependent on the variability of the scene and hence will provide an inconsistent enhancement depending on background and dimension of the region. Spatial contrast reduction, which takes advantage of the blurring effect of dust over otherwise sharply-defined terrestrial features, has been applied successfully for dust detection over land, but often fails over laminar ocean backgrounds and is limited over land in regions where the terrain is less variable (e.g., desert plains, where dust incidentally is most common).
None of the previous methods enlist multi-channel visible data to take advantage of the inherent spectral variability of dust at these wavelengths, due primarily to the unavailability of such data at the times of those writings. The unified land/ocean Dust Enhancement Product is a novel solution to the daytime dust detection problem. In comparisons between the visible/infrared combination technique (e.g., as is applied to Meteosat 5 data) and the current algorithm (applied to Terra-MODIS data) for a space and time co-located dust storm event in Southwest Asia has revealed superior detection of dust by the current method over both land and water. As such, the new technique is thought to be of immediate relevance to Navy Meteorology/Oceanography (METOC) operations in several regions of the world that experience significant dust outbreaks (including Southwest Asia, Africa, the Mediterranean, and the West Pacific).